On a solo fishing holiday in Cornwall, Harley Street doctor Paul Marten finds his line caught and is quickly pulled deep underwater. Waking in a cave, he finds himself under the care of - or perhaps held hostage by - a bewitching mermaid, Miranda.

With no eligible men in the underwater world, Miranda explains her desire for a land-bound mate, and her intention to keep him there accordingly. Paul protests and explains his own love for his wife Clare, so Miranda gives him the option of help to return to land in exchange for taking her with him back to London and helping her to see all the sights.

Paul duly takes up the offer, and it's not long before the pair are headed for the capital. However, his forewarning to Clare of their new house-guest has left her expecting a half-decrepit old dear, not a stunning young woman! Miranda, never the less, is perfectly pleasant and asks for very little - although she soon has all the other men in their lives, notably valet/chauffeur Charles, and friend Nigel, eating out of her hand!